1) Starting with an existing GCC Cortex M3 project - the kernel and port layer [the files in the FreeRTOS/Source/ directories) will be the same for any Cortex M3 device, so anything in the Source directory or subdirectory can be left as is.
2) Look at the SAM3 IAR demo project to see which files it includes from the FreeRTOS/Demo directory and subdirectories.
3) Replace the files from the FreeRTOS/Demo directory tree that are referenced from the GCC project with those from the FreeRTOS/Demo directory tree that are referenced from the existing SAM3 IAR project.